1. Field of the Invention
This invention relates to an improved infrared seeker assembly and particularly to one having an improved coldfinger assembly construction.
2. Discussion
Infrared detection systems are often used in conjunction with munitions and night vision systems for sensing electromagnetic radiation in the wavelength range of one to fifteen micrometers. Because many such detection systems have detector arrays which are most sensitive when operated at about 80.degree. K., a cooling system is required to produce and maintain the required low operating temperatures. Typically, such cooling systems either take the form of a cryostat operating using the Joule-Thompson effect, or a Stirling cycle cryoengine. the cooling systems are used in conjunction with an evacuated dewar in which the infrared detector is placed. The dewar is evacuated to remove thermally conductive gases which would otherwise occupy the area surrounding the detector so that potential heat loss through convection and conduction is minimized. The evacuated dewar also prevents moisture from condensing on the detector. The infrared detector is typically cooled by placing an indented region ("coldwell") of the dewar in contact with an expansion chamber ("coldfinger") of the cryogenic cooling system. The coldfinger is a cylindrical tube having an end which is cooled and which supports the detector and related components. The cooling systems produce cyclical cooling by sequential compression of a working fluid such as helium, removal of the heat generated during compression of the working fluid, and subsequent expansion of the working fluid within the coldfinger. Thermal energy is withdrawn from the detector through the optical mounting platform ("end-cap") which is in thermally conductive communication with the coldfinger. Since the detector is in thermal communication with the coldfinger, expansion of the working fluid within the coldfinger cause thermal energy to be withdrawn from the detector.
In order to produce efficient conductive withdrawal of thermal energy from the infrared detector, the end-cap on which the detector is mounted must be fabricated from a material possessing specific metallurgical properties. Ideally, these properties include high strength, a high modulus of elasticity and high thermal conductivity. Additionally, the end-cap material must be capable of being permanently bonded to a thin-walled tubular coldfinger to produce a low distortion, hermetically sealed coldfinger assembly.
A number of design constraints affect the design of the dewar/coldfinger assembly. Since the coldfinger tube is a cantilever supported cylinder, it must have sufficient bending stiffness to control deflection of the infrared detector. Such requirements become particularly significant when the infrared seeker assembly is used as part of munitions subjected to intense vibrations and high levels of boost-phase acceleration. Another significant design parameter is the extent to which heat is transferred from the warn end of the coldfinger cylinder to its cold end. Reductions in heat transfer rate allow the capacity of the cryogenic cooling system to be reduced. Unfortunately, reductions in the cross-sectional solid area of the coldfinger tube for reducing heat transfer adversely affects its bending stiffness for a given material and tube diameter. Another design consideration is the cool-down rate for the cold end components of the coldfinger assembly. Since infrared seekers are often used in expendable munitions which must acquire a target soon after (or before) their launch, cool-down time becomes a critical consideration for some applications.
Prior art dewar/coldfinger tubes have been formed from various materials. Glass has been used since it has low thermal conductivity, but, unfortunately, does not yield low conductance coldfingers because it is too fragile when it is made in decreased thicknesses desired to minimize conduction heat loads. Examples of such glass dewar coldfingers are described by U.S. Pat. Nos. 3,851,173 and 3,719,990. Other art coldfingers have been made form various metals, for example, as described by U.S. Pat. No. 4,528,449. Meals are less fragile than glass but often cannot be made thin enough to obtain lower heat loads than glass due to their higher thermal conductivity. Plastic materials have also been used for non-evacuated detector units and have obtained low heat loads. However, due to their porosity, plastic coldfingers have not been effectively used in vacuum type dewars without the need to electroplate a thin metallic skin over the exterior surface of the tube.